Wafer polishing method and apparatus

ABSTRACT

A wafer polishing apparatus includes a wafer polishing assembly having a plurality of wafer carriers for substantially simultaneously polishing a plurality of wafers against a rotating polishing surface. A plurality of wafers to be polished are substantially simultaneously loaded into the plurality of wafer carriers by wafer holding apparatus of an index table. Similarly, a plurality of wafer carriers are substantially simultaneously unloaded into wafer holding apparatus of the index table. The wafer carriers are individually computer controlled for exact polishing and different polishing requirements can be met at the same time by different wafer carriers.

BACKGROUND OF THE INVENTION

This invention relates to polishing methods and apparatus and moreparticularly, to such methods and apparatus for accurately polishingwafers of semiconductor material with high throughput and in a mannercompatible with semiconductor processing clean room environments.

The production of integrated circuits begins with the creation of highquality semiconductor wafers. Each wafer is of relatively high cost dueto the detailed processing needed to produce it. During the integratedcircuit production process, an extremely flat surface is desired on atleast one face of the wafer. Wafer polishing to achieve such a flatsurface is a known technique.

Such polishing generally includes attaching one side of the wafer to aflat surface of a wafer carrier or chuck and pressing the wafer againsta flat polishing surface. The polishing surface is moved under thewafer, and the wafer may also be rotated about its vertical axis andoscillated back and forth to improve polishing action. The polishingsurface is generally a pad attached to a rigid flat table which isrotated to provide movement and onto which an abrasive and/or chemicalslurry is pumped. The joint functions of the pad, the slurry, and therelative movements of the components produces a combined mechanical andchemical process at the wafer surface which produces a highly flatsurface on a wafer where surface variations are kept to less than, forexample, 0.5 μm.

Polishing has typically been performed prior to integrated circuitfabrication so that a flat surface is available on the semiconductorwafer on which the circuit fabrication can take place. As integratedcircuits increase in complexity, the conductive line widths have reducedconsiderably, making the focus and depth of field of the imaging processmore sensitive to surface variations on the substrate. This hasincreased the desire for wafers with improved surfaces. Further duringthe integrated circuit fabrication process, layers of, for example,conductors and dielectrics, are built up on the wafer, on top of whichother such layers are to be created. Thus, it has become necessary to"re-flatten" the wafer surface during the actual fabrication of theintegrated circuit and not merely before it. The act of re-flattening isreferred to as planarization. At each successive one of severalplanarization operations the wafer is considerably more valuable. Givensemiconductor processing costs, it is quite possible that a single 8"partially processed wafer is worth $10,000 or more when planarization isperformed. Great care in handling of each such wafer is obviouslyrequired.

Speed of wafer polishing has always been of interest but has become moreimportant when planarization is one of the necessary sequentialprocessing steps. Prior arrangements, typically, polish one or twowafers, with substantial waiting time to load and unload wafers. Methodsand apparatus are needed to speed up the polisher process.

The increase in value of the wafers being polished has greatly increasedthe need for precision in the planarization process. Improper polishingof a wafer worth $100 is a completely different matter than improperlypolishing one worth $10,000. Methods and apparatus are needed to provideimproved polishing, particularly in a rapid production environment.

These needs are met by the present invention.

SUMMARY OF THE INVENTION

Wafer polishing apparatus in accordance with the present inventioncomprises a polishing assembly having a plurality of wafer carriers forsubstantially simultaneously engaging a plurality of wafers of materialwith a polishing surface. The apparatus includes an index table forholding wafers to be polished, and positioning apparatus to move thepolishing assembly between the polishing surface and the index table. Atthe index table, all wafer carriers of the polishing assembly aresubstantially simultaneously loaded with wafers. After loading thecarriers, the polishing assembly is positioned in polishing engagementwith the polishing surface. By incorporating an index table into theapparatus, unpolished wafers can be loaded onto the index table inpreparation for loading them simultaneously onto the wafer carriers,providing throughput advantages.

The index table indexes in increments when being loaded with unpolishedwafers so that the wafers can be placed thereon one at a time, asretrieved from a multi-wafer cassette. The movement of unpolished wafersto the load cups advantageously occurs while the polishing assembly isat a polish position polishing another plurality of wafers. Uponcompletion of polishing, the assembly returns to the index table toreceive substantially simultaneously another set of wafers to bepolished.

The index table may also comprise a plurality of unload cups which areused in a similar manner to the load cups to substantiallysimultaneously remove polished wafers from the wafer carriers afterbeing polished. The removal of polished wafers from the unload cups canthen be performed while other wafers are being polished by the polishingassembly.

The alignment of polish assembly, index table and polishing surface ismaintained by providing a stable framework in the apparatus. To thisend, a linear track for moving the polishing assembly extends betweenthe polishing surface and the index table. The linear track provides astable, rugged frame while permitting controlled movement of thepolishing assembly between the index table and the polishing surface.

The apparatus may also include an automatic arrangement for washing eachwafer as it is removed from the index table. Such washing assures thatthe polished wafers removed from the apparatus are suitable for a cleanroom environment.

The apparatus is controlled by a computer which processes many separatefeedback loops to maintain the accuracy of operations. For example,polishing pressure is applied at each wafer carrier by an air cylinderand applied pressure is sensed by a pressure sensor of each wafercarrier. Oscillation and rotation of each wafer carrier is provided byseparate servo motors, the position and rotation rate of which is alsosensed. Ranges of values for desired pressure and wafer carrier motionare established based on operator input. The computer then reads actualoperating parameters measured by the sensors and adjusts the airpressure and servo motor motion to keep the actual parameters within thedesired ranges.

The operator enters data indicative of operating parameters for each ofthe wafer carriers being used. These parameters then form the basis ofthe desired ranges which are separately stored in the computer.Advantageously, the operator can establish the same or differentparameters for each wafer carrier. Since each wafer carrier iscontrolled by the computer in accordance with variables stored for thatwafer carrier, the apparatus can differently process wafers on separatewafer carriers.

Each wafer carrier of the preferred embodiment includes an upper forceconveying member having a central axis for conveying pressure forcesalong the central axis and rotational forces about that central axis. Apolishing member of the wafer carrier comprises a flat lower surfacehaving a polishing axis. Pressure forces are coupled between the forceconveying member and the polishing member by a force coupling memberincluding a first race member symmetrically disposed about the centralaxis of the force conveying member, a second race member symmetricallydisposed about the polishing axis of the wafer carrier, and ballbearings held between the first and second race members. The first racemember, the ball bearings, and the second race member cooperate to focuspressure forces through the force coupling member to a point on thepolishing axis. Further, rotational forces are conveyed by a pluralityof cam followers disposed about the periphery of the force conveyingmember which abut bearing surfaces on the polishing member, to couplerotational forces. After the force conveying member is inserted into acylindrical opening in the polishing member, it is held in placeresiliently by a collar which includes a plurality of springs forholding the force conveying member in the cylindrical opening of thepolishing member, to maintain pressure on the ball bearings.

A lower flat surface of the polishing member includes a plurality ofholes therethrough which communicate with a central passage into theforce conveying member. This hollow passage is sealed by flexible meansto permit relative motion of the polishing member and the forceconveying member, while providing a substantially fluid-tightcommunication channel.

The polishing member also includes a lip around its polishing surface toprovide additional support for wafers carried thereby. In the preferredembodiment, the lip comprises a ring of material having threads on aninner surface thereof, which engage with threads around the outersurface of the polishing member. The height of the resulting lip can becarefully adjusted by controlling the depth to which the threads of thering and the polishing member are engaged. Advantageously, a collar isapplied over the ring, which collar frictionally engages the ring tokeep it from rotating and becoming misadjusted.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Figures of the drawing, like reference numerals identify likecomponents, and in the drawing:

FIG. 1 is a perspective view of a wafer polishing system embodying thepresent invention;

FIG. 2 is a plan view of the system of FIG. 1 with the top thereofremoved;

FIG. 3 is a sectional view of the wafer polishing system through anindex table thereof;

FIGS. 4 and 5 are additional plan views of the system illustratingdifferent parts of the wafer polishing process;

FIGS. 6a and 6b are sectional views illustrating wafer loading;

FIGS. 7a and 7b are sectional views illustrating wafer unloading;

FIG. 8 is a side view illustrating a wafer polish assembly and itsmotion within the system;

FIG. 9 is a plan view of the wafer polishing assembly;

FIG. 10 is a sectional view of a polish arm assembly included in thewafer polishing assembly;

FIGS. 11 and 12 are side and top views of a wafer cleaning assembly;

FIG. 13 is a sectional view of a wafer carrier which is a part of thewafer polishing assembly;

FIG. 14 is a perspective view of a lower force member which is part ofthe wafer carrier of FIG. 13;

FIG. 15 is a block diagram of control apparatus for the wafer polishingsystem;

FIG. 16 is a sectional view of the wafer polishing system sectionedthrough a polishing table thereof; and

FIGS. 17-22 are flow diagrams of process control for the wafer polishingsystem of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a perspective view of a wafer polishing apparatus 100embodying the present invention. Wafer polishing apparatus 100 includesa wafer input/output module 101 and a wafer process module 102. Thewafer polishing apparatus 100 is constructed so that input/output module101 can be placed, for example, inside a class 10 clean room environmentwhile the process module 102 is placed beyond an adjacent wall perhapsin a class 1000 clean room environment. By means not specifically shown,air flow is created within the polishing system and the air pressuresare regulated such that the environment of the class 10 clean room isnot negatively influenced.

FIG. 2 is a top view of wafer polishing apparatus 100 in which the topand certain other structure of the perspective view has been removed forease of understanding. Additionally in FIG. 2, a wall 104 is representedshowing separation of input/output module 101 and process module 102thereby. Wafers are presented to and removed from input/output module101 by means of multi-wafer cassettes, of which two input cassettes 106and 107 and two output cassettes 108 and 109 are shown in FIGS. 1 and 2.Cassettes 106 through 109 are of a type known in the art which store insubstantially horizontal orientation up to 25 wafers of a pre-chosendiameter. In the present description, 8" wafers are discussed. Each ofthe cassettes 106 through 109 has closed side and rear portions, with anopen front portion for the loading and unloading of the wafers.Input/output module 101 also includes a 3-axis loading robot 111 which,by means known in the art, removes wafers from cassettes 106 and 107 oneat a time and places them on an aligner unit 113. Loading robot 111 may,for example, be a Model 351 by ADE and aligner 113 may, for example, bea Model 428 by ADE. Aligner unit 113 centers the wafer presented theretoby robot 111 and positions it for the reading of a bar code on thewafer. After alignment of the wafer, an input gripper 115 grips theedges of the aligned wafer.

Processing module 102 includes an index table 117 which is used toreceive wafers from, and provide wafers to, input/output unit 101. Indextable 117 comprises a rotatable annular ring 118 including five waferunload cups numbered 119 through 123 and five wafer load cups numbered124 through 128. Unload cups 119 through 123 are disposed at 72°increments about the vertical center axis of index table 117, and loadcups 124 through 128 are similarly disposed at 72° increments about thevertical axis in alternating positions with the unload cups. Thus, awafer cup is present at 36° increments about the rotatable member 118,and the load and unload cups are alternatingly disposed.

Index table 117 can be rotated through 360°, and is primarily rotated ininteger multiples of 36° in the counterclockwise direction (FIG. 2) toposition wafer cups 119 through 128 for input/output operation and toload and unload wafers in groups of five to and from a polish assembly132. Any indexing in the clockwise direction is specifically identifiedherein. Two positions of index table 117 are identified in FIG. 2. Oneposition 129, called the input position, occurs when table 117 has aninput cup adjacent to the input gripper 115. In FIG. 2, load wafer cup124 is in input position 129. A second position 131, called the outputposition, occurs when table 117 has an unload cup, adjacent to an outputgripper 116. In FIG. 2, cup 120 is in the output position 131. In thepresent embodiment, all loading and unloading of wafers to and frompolish assembly 132 occurs with wafer cups of the type in the inputposition, that is, load functions of assembly 132 are performed when aload cup is in the input position 129 and unload functions are performedwhen an unload cup is in the input position 129. To implement loadingand unloading of the wafer cups, an air cylinder 159 is disposed underindex table 117 in a position to engage the wafer cup at the inputposition 129. Four additional air cylinders 159 are disposed similarlyunder index table 117 at 72° increments from the input position. FIG. 3is a representation of the process module 102 taken along section line3--3 and shows a sectional view of index table 117 and certainassociated apparatus.

When a wafer has been aligned by aligner 113, and an empty load cup 124,is present at input position 129, input gripper 115 grips the alignedwafer and rotates it vertically 180° to place the newly aligned wafer ininput cup 124, as shown in FIG. 4. After cup 124 has received a wafer,the index table 117 is rotated by 72° counterclockwise under the controlof an index drive system 130 driven by AC servo motor 131 (FIG. 3) toplace the next available load cup, e.g. 128, in the input position 129to receive an aligned wafer. Index table drive system 130 operates underthe control of computer 103 to perform its indexing operation. Byalternatingly loading load cups and indexing index table 117, all fiveload cups 124 through 128 will be loaded with wafers awaiting polishingand load cup 124 is again in the input position 124. No further inputoperations are performed until the five load cups have been emptied asis described below.

In the present embodiment, wafers are polished five at a time by amulti-head wafer polish assembly operating in conjunction with arotating polishing table 134. The multi-head polish assembly 132 isshown in cutaway view in FIG. 1 and is represented in FIGS. 2, 4 and 5as a transparent decagon. The details of construction of polish assembly132 are provided later herein. Polish assembly 132 includes five wafercarriers 139-143 and is capable of simultaneously pressing five wafersonto rotating polishing table 134 while at the same time rotating eachwafer and oscillating each wafer back and forth between twocircumferences on rotating polish table 134. In FIGS. 2, 4 and 5, thewafer carriers 139-143 are represented by circles having wide darkenedperimeters. The two circumferences between which wafer carriers 139 and143 can oscillate consist of a home position as shown in FIG. 2 and amaximum outward oscillation position as shown in FIG. 4. When waferpolishing is completed or when the wafer carriers 139 through 143 areempty, they are raised to a significant height above polish table 134.When the wafer carriers 139 through 143 are raised, they are alsobrought into the home position of oscillation.

When index table load cups 124 through 128 each contain a wafer to bepolished, those wafers must be transferred to the wafer carriers 139through 143 before polishing can begin. The loading of wafers into wafercarriers 139-143 begins with the movement of polish assembly 132 from aposition over the polish table 134 to a position over the index table117. As shown in FIGS. 3 and 8, polish assembly 132 is attached to amain structure plate 136 of process module 102 by means of a pair oftransport rails 137 running the length of process module 102 betweenrotating polish table 134 and index table 117. Polish assembly 132 isconnected to rails 137 by means of a transport frame 144 which isconnected to rails 137 by four transport linear bearings, such as THKbearings No. HSR 35CB2UU. Linear motion along rails 137 is controlled bya motor driven transport ball screw such as THK No. BLK 3232EZZ, whichis driven by an AC servo motor 165. FIG. 5 shows the position oftransport head polish assembly 132 over index table 117 at thecompletion of linear motion from polish table 134. The loading operationbegins when the five wafer carriers 139 through 143 are lowered into atub 133 formed at the center of the annular ring 118 and they arerotated against a brush 146, while at the same time being sprayed by asolvent, such as water, from a plurality of nozzles 147. Wafer carriers139 through 143 are then raised to their maximum upward position andoscillated outward to their maximum outward position. By prealignment ofwafer carriers 139-143 on polishing assembly 132 and proper indexing ofthe rotation of index table 117, each wafer carrier 139 through 143 isabove and in substantial vertical alignment with one of the cups ofindex table 117. When a load operation is to be performed, table 117 ispositioned (FIG. 4) so that load cups 124-128 are in vertical alignmentwith wafer carriers 139 through 143 while, when an unload operation isto be performed (FIG. 5), index table 117 is positioned to providevertical alignment between unload cups 119-123 and the wafer carriers139 through 143.

In the present description, we will assume that wafer carriers 139through 143 have just completed a polish operation and each contains awafer to be unloaded. FIG. 4 shows the relative position of index table117 after receiving wafers from input/output module 101. In order toalign unload cups with wafer carriers 139 through 143 for the unloadingoperation, index table 117 is indexed by one 36° step clockwise,resulting in the position of load and unload cups as shown in FIG. 5.Upon such proper positioning, an unload wafer operation is substantiallysimultaneously performed between unload cups 119 and 123 and respectiveones of wafer carriers 139 through 143. Upon completion of such anunload operation, wafer carriers 139 through 143 may be returned to thehome position and lowered again for additional cleaning by brush 146 andnozzles 147. When the carriers 139-143 are to be loaded with new wafersfor polishing, they are raised and oscillated to their maximum positiononce again and index table 117 is rotated by a 36° counterclockwiseincrement so that load cups 124 through 128 are in vertical alignmentwith wafer carriers 139 through 143. Once such alignment is achieved, acarrier load operation is performed to substantially simultaneously loadall five wafer carriers. Also, as shown in FIG. 5, while the multi-headpolish assembly is engaged in loading and unloading operations over theindex table 117, the rotary polishing table 134 may be renewed by meansof an abrasive pad treatment arrangement 149. Pad treatment arrangement149 consists of a rotating head 150 having an abrasive on its lowersurface. The rotating head is oscillated back and forth across thepolish table 134 to prepare the surface for another polishing session.The surface preparation member 149 carries rotating head 150 on anoscillating member 151 supported at a pivot point 152.

The wafer carrier load operation with respect to an exemplary wafercarrier 139 is illustrated in FIGS. 6a and 6b. Each load cup, e.g. 124,comprises a movable insert 154 and a support member 155 therefor.Support member 155 comprises primarily a flat supporting surface ofrotatable member 118 with an aperture therethrough to permit upward anddownward travel of the cup insert 154. The cup insert 154 is made of amaterial such as Delrin™ to be gentle to the wafers being handled, andincludes an angled surface 156 having an uppermost inner diametersomewhat larger than the outer diameter of the lowest point of wafercarrier 139 and a lower inner diameter substantially equal to the outerdiameter of wafer carrier 139. The angled surface 156 of the wafer cupprovides self-guiding alignment between the wafer cup and the bottom ofa wafer carrier during load and unload operations. Each load cup 124-128also includes a bottom member 157 for engagement with a piston 158 ofair cylinder 159. The thickness of bottom member 157 and the travel ofpiston 158 is such that, when the piston is actuated, as represented inFIG. 6b, the cup insert will be driven upward onto the lower face ofwafer carrier 139 so that the wafer carried by cup insert 154 issubstantially in contact with a lower flat surface 261 of wafer carrier139. When piston 158 is at its upper travel position, a vacuum ispresented through holes in the flat lower surface 261 of wafer carrier139 to secure the wafer onto the surface. Thereafter, air cylinder 159is deactivated, lowering the cup insert 154 into cup support member 155.Prior to lowering cup insert 154 it may be desirable to perform a vacuumtest to assure that the wafer in each cup insert 154 has been secured towafer carrier 139.

FIGS. 7a and 7b represent the similar process of unloading a wafer fromexemplary wafer carrier 139. Unload cup 120 includes a cup insert 161having substantially the same upper properties and dimensions as cup 154of load cup 124. The bottom member 162 of unload cup insert 161 is,however, positioned slightly farther from the top of piston 158 of aircylinder 159. This slightly larger spacing than in the load cups resultsin cup insert 161 being moved upwardly to a slightly lower positionunder wafer carrier 139. When in the upward position, vacuum to thesurface 261 of wafer carrier 139 is terminated and the wafer is allowedto separate from wafer carrier 139. It may be desirable to provide apositive flow of fluid such as air or water to force the wafer from theface of carrier 139. The positioning of cup insert 161 permits the waferto drop for a small distance, represented by 163, before being caught bythe cup insert. This distance assures that the wafer has separated fromsurface 261 of wafer carrier 139. A vacuum test may be performed toassure that the wafer has actually separated from the face of wafercarrier 139. On the completion of the unload sequence, air cylinder 159is deactivated and insert 161 returns to its resting position on thesurface 155 of index table 117.

After polished wafers have been placed in unload cups 119-123 andunpolished wafers have been loaded into wafer carriers 139 through 143,the wafer polish assembly 132 is moved along rails 137 to a positionover rotary polish table 134. FIG. 8 shows in side view, the movementalong the side rails 137. FIG. 8 includes a dashed representation of atransport frame 144' in the left-hand or index position, and a secondsolid line representation of transport frame 144 shown in the right-handor polish position. It is to be noted that only one such transport frameis present in the embodiment, both being shown to indicate the range oflinear movement of the polish assembly 132. When in the polish position,four wedges 166 carried by transport frame 144 engage respective slots167 in associated support members 168 attached to main plate 136. Twosuch wedges are shown in FIG. 8, the other two being carried on theopposite side of transport frame 144. Upon engagement between wedges 166and slots 167, four solenoids 169 are actuated to rotate a roller-endedlever arm 171 into engagement with wedges 166, to maintain a tightlyfitting relationship between wedges 166 and slots 167. By the operationof wedges 166 and support members 168, upward forces created by thepressure of wafer carriers 139-143 on the table 134 are borne by thesupport members 168 and are not by bearings 145. Polish assembly 132 ismoved along rails 137 in response to the rotation of transport ballscrew 163, which is driven by AC servo motor 165 in response to commandsfrom computer 103.

Wafer polishing is accomplished by the combined action of the wafercarriers 139-143 of polish assembly 132, and the motion of polishingtable 134 operating in the presence of an abrasive and/or chemicalslurry. FIG. 16 shows a sectional view of process module 102 alongsectional line 16--16, through polishing table 134. Polishing table 134is rotatably supported on a central shaft 282 above main structure plate136 by a bearing member 281. Shaft 282 extends through plate 136 and isconnected by a drive belt 283 and pulley 284 to an output pulley 285 ofpolish table motor 280. Motor 280 operates in response to commands to aninterface 442 of the type well known in the art, to closely regulaterotational speed of polishing table 134. FIG. 16 also shows a pair ofslurry nozzles 221 which distribute slurry onto table 134 from a slurrypump 223 (FIG. 15) operating under the control of computer 103.

Table 134 comprises a disc-shaped upper surface 286, which is carried bya support frame 288, for supporting the upper surface 286 and definingat least one cooling fluid chamber 293. Shaft 282 has a hollow channel291 along its central axis and includes a tube 290 disposed therein todefine two fluid channels. One fluid channel is within tube 290 and thesecond is in the annular spacing between tube 290 and the inner surfaceof channel 291 during operation. Cooling fluid is pumped via centraltube 290 and a fitting 297 into channel 293. Warmed water from channel293 flows through the annular channel around tube 290 and is returned toa heat exchanger 295 (FIG. 2) through fitting 297. Heat exchanger 295,which includes a fluid pump (not shown), continues to circulate and coolthe operating fluid to maintain a reduced temperature at polish table134.

The polishing assembly 132 shown in plan view in FIG. 9 includes fiveindependent polishing units, each in a separate zone defined within thepolish assembly. The structure of assembly 132 comprises an upper steelplate decagon 170 separated from a lower parallel steel plate decagon172 by a central steel support member 174 and five zone-defining steelplates 175, as shown in top view in FIG. 9. The support member 174 iswelded to the upper and lower plates 170 and 172, and each of thezone-defining plates 175 is welded to the length of the support member174 and to the upper and lower plates 170 and 172. An oscillating polisharm 180 is pivotally mounted within each zone of the polish assembly 132to oscillate horizontally about a vertical axis through a point 176. Inresponse to control signals from computer 103, oscillating polish arm180 regulates the position of one wafer carrier, e.g. 139, its pressureon rotating polish table 134 and the rotation rate of the wafer carrier139.

An oscillating polish arm 180 is shown in detail in FIG. 10. Polish arm180 comprises a vertical pivot column 181 to which is welded an upperhorizontal support member 182 and a lower horizontal support I-beam 183.The free ends of member 182 and I-beam 183 are connected by an endmember 184. The upper end of pivot column 181 is bolted to the rotatingsurface of a rotational speed reducer 186, which extends through anaperture in upper plate 170. In the present embodiment, speed reducer186 is a Dojen speed reducer Series II, Model Number 04. The stationaryportion of speed reducer 186 is bolted to the upper surface of plate170. The lower end of pivot column 181 is supported by a bearing 187 andbearing support pin 188 attached to the lower plate 172 of housing 132.An AC servo motor 190 is connected to and drives speed reducer 186. Byselectively energizing servo motor 190 to rotate clockwise orcounterclockwise, the oscillation of the polish arm 180 about thevertical axis defined by column 181 is readily controlled.

The polish arm 180 supports the apparatus which controls the function ofone wafer carrier, e.g. 139. The raising, lowering and downward forceson the wafer carrier are controlled by a double acting air cylinder 192which is attached to the upper surface of polish arm upper member 182.Air cylinder 192, which may, for example, be a SMC Series NCA1 extendsthrough an arcuate slot 195 formed in upper plate 170 so that freeoscillation of arm 180 is not prevented. An output shaft 194 of aircylinder 192 is attached to a circular flange 196, which is connected toa cup-shaped member 197. Cup-shaped member 197 receives a bearing collar198 through a circular aperture 199 in the cup-shaped member. The unionof flange 196 and cup-shaped member 197 form a cylindrical chamberhaving larger dimensions than the flanged top portion of bearing collar198, so that no transverse forces are conveyed from beneath the bearingcollar 198 to the air cylinder 192. A bottom surface of bearing collar198 is attached to a top surface of a force sensor 202 such as aSensetel Model 41 loadcell, the bottom surface of which is attached to ahollow cylindrical force-conveying member 204. Force conveying member204, which is connected at a bottom surface thereof to the periphery ofa hollow carrier driving shaft 206 by a thrust bearing 208. Internal tohollow force-carrying member 204 is a fluid coupling 210 which, via anaperture 209 in force-conveying member 204, communicates fluids andvacuum to the hollow center of carrier driving shaft 206 via a fluidconnection 211.

Carrier driving shaft 206 is supported at I-beam 183 by a ball splineand bearing assembly at 212, which holds shaft 206 from lateral movementbut which permits upward and downward movement as well as rotation ofthe shaft. Assembly 212 comprises a ball spline collar 214, such as THKLBST50, which is held in place by a bearing 216, such as Torrington9120K. The bottom end of driving shaft 206 is attached to a circularflange 218, which extends through an arcuate slot 219 in the bottom ofplate 172 of assembly 132. Arcuate slot 219 is substantially identicalto arcuate slot 195 and is present to permit oscillation of shaft 206and carrier 139. The top of ball spline 214 is connected to a gear 224,which is rotationally driven by a gear 226 attached to an output shaft227 of a speed reducer 222. An AC servo motor 220 provides rotationalforces to speed reducer 222 and thus to shaft 206 under the control ofcomputer 103.

FIG. 13 is a sectional view of a wafer carrier 139 constructed to evenlydistribute downward pressure forces and rotational forces from shaft 206to a wafer carried by the wafer carrier. Wafer carrier 139 comprises anupper force-conveying member 251 of circular, horizontal cross-sectionwhich is bolted to the lower surface 219 of flange 218. Upper member 251is received by a cylindrical upper opening of a lower force-conveyingmember 253 which is also shown in perspective view in FIG. 14. The outerdiameter of upper member 251 is smaller than the inner diameter of thereceiving cylinder of lower member 253, to permit alignment variationsbetween the axis of driving shaft 206 and the axis of rotation of wafercarrier 139. The coupling between upper member 251 and lower member 253comprises a bearinged gimble to maintain evenness of pressure across theflat surface 261 of the carrier 139 in the face of alignment variations.

Downward pressure forces are conveyed by a ball bearing assemblyincluding a plurality of ball bearings 258, supported by a lower race255 and retained by a retainer 257. As shown in FIG. 13, lower race 255is attached to lower force member 253 about its central vertical axis,and includes a groove 260 for aligning the ball bearings 258. Pressureforces are applied to ball bearings 258 by an upper race 256, which issymmetrically attached about the vertical axis of the lower surface ofupper force member 251. The bearing contacting surface of upper race 256is shaped in the cross section shown, to have a radius R substantiallyequal to the distance to a predetermined point on the central verticalaxis of lower member 253. In the present embodiment, the predeterminedpoint is at the face-defining surface 261 of the wafer carrier 139, andis labeled 262. This configuration focuses the applied pressure forceson the center of the surface 261 for even force distribution. Theconfigurations of races 255 and 256 may be chosen to raise or lower theforce focus point from that shown; however, it is most desirable tofocus the forces on the vertical axis.

Rotational forces are coupled from upper member 251 to lower member 253by four cam followers 263, which are attached at 90° spacing around thecylindrical periphery of upper member 251. The outer rings of camfollowers 263 are disposed in slots 265 (FIG. 14) which are spaced at90° angles around the upright cylindrical portion of lower member 253.The slots, which are slightly wider than the diameter of the camfollower 263 outer rings for freedom of movement, are sufficiently longvertically to permit foreseeable ranges of required movement. Rotationalforces are conveyed to lower member 253 when cam followers 263 abut theside (bearing) surfaces of the slots 265. Upper member 251 is heldwithin lower member 253 by a collar 267 which is fastened to the lowermember 253 after the insertion of the upper member 251 therein. In orderto maintain pressure on ball bearings 258 and to allow freedom ofmovement between members 251 and 253, collar 267 includes a plurality ofsprings 269 which maintain a flexible downward pressure on upper member251 from collar 267.

Lower member 253 is produced in two sections so that fluid and vacuumcan be communicated therethrough to surface 261. An upper section 271 ofthe lower member 253 has a plurality of channels 273 milled thereinwhich communicate with a central aperture 272. The surface-defininglower section 274 of lower member 253 includes a plurality of holes 275drilled therethrough for communication of fluids and vacuum betweensurface 261 and the milled channels 273. A cavity 274 is formed betweenthe upper member 251 and flange 218, which cavity is sealed at its lowersurface by a flexible gasket 276. Any fluid or vacuum which iscommunicated in the hollow center of drive member 206 is passed viacavity 274 to the holes in surface 261 by a channel 277, aperture 272,milled channels 273 and the holes 275 through surface member 274. Thewafer carrier 139 also includes a hollow cylindrical ring 268 of plasticmaterial, such as Delrin, which is disposed over surface member 274 toform an outer lip 270 for surface 261. The lip 270 is used to hold anattached wafer from sliding on surface 261, and the optimum height oflip 270 varies depending on the wafer thickness and other processvariables. As shown in FIG. 14, an outer flange 262 of lower member 253includes threads 264 on the outer surface thereof which mate withthreads 264' on the inner cylindrical surface of ring 268. By threadingring 268 onto flange 262, the height of lip 270 can be finely adjustedby rotating the ring. When the desired height of lip 270 is achieved, itis frictionally engaged by a retaining ring 266 bolted to the lowermember 253. Advantageously, scribe marks 259 can be placed aroundretaining ring 266, which marks can be compared to a reference line 254on ring 268 during adjustment of lip 270 height.

When the polish assembly 132 is at the polish table 134, the waferoutput process of moving polished wafers from unload cups 119-123 ofindex table 117 into an output wafer cassette, e.g. 108, can take place.The wafer output process begins by placing index table 117 in a positionin which unload cup 120 is in the output position 131. The outputprocess begins when the wafer cup 117 at the output position is raisedby an air cylinder 160 and unload gripper 116 edge grips the wafer 235in unload cup 120, rotates it vertically and places it in water cleaningapparatus 230. Water cleaning apparatus 230 is shown in detail in sideview FIG. 11 and top view FIG. 12. The unloaded wafer 235 is placed bygripper 116 on four spindles 232, each having a ball bearing mounted cap233. The spindles 232 and caps 233 are positioned to support theperimeter of output wafer 235 on a ledge 236 of all four caps. A washassembly 237 including six revolving brushes 238 is then driven to theright toward wafer 235 along guide shafts 240 to a position above andbelow the wafer. Upon such positioning, a bottom brush-carrying portion241 of wash assembly 237 is moved upwardly by an air cylinder 239 toengage wafer 235 between the upper and lower sets of brushes 238. Thebrushes are then rotated by stepper motors 247 and 246 and belts (notshown), while deionized water is applied by a plurality of nozzles 243in upper member 244 of wash assembly 237 and by a plurality of nozzles245 mounted under the wafer 235. The asymmetrical placement of brushes238 rotates the wafer 235 in the water, thereby cleaning its surface.After a preset time for completion of cleaning, washer assembly 237 isreturned to its leftmost position and wafer 235 is raised by anelevator/arm apparatus 249 to a position above the water cleaningassembly 230. Elevator/arm assembly 249 is then moved along guide member248 to a water slide 250 (FIG. 1), where the wafer 235 is released toslide by water flow into output wafer cassette 108. Advantageously, thewafer cassette 108 is kept submerged in water until being removed by anoperator.

The method and apparatus described herein is controlled by the computer103, which includes an INTEL 486 main processor, memory, and suitableinput/output interfaces for controlling and sensing productionprocesses. The computer assembly, which may be a VME Bus System, and itsinterface to production processes, are well known in the art and are notdescribed in detail herein. Also, each of the servo and stepper motorsdescribed includes an associated position and/or rate sensor which isused by the computer 103 in closed loop feedback control of the rotationand position of the motor. Such position and rate sensors are also wellknown in the art. Further, although the computer 103 is capable ofcommunicating with a process control master computer (not shown) whichmay be in control of an entire wafer production process, such mastercomputer or communication is not needed for the present method andapparatus and is therefore not described herein.

FIG. 15 is an electrical block diagram of the present apparatus showingcontrol as exercised by computer 103. Most control is performed inclosed feedback loops by sending a command on buses 450 and 450' fromcomputer 103 to an action device and checking a sensor by the computervia buses 451 and 451', to assure that the commanded act was correctlyperformed to achieve a desired result. In FIG. 15, dashed lines areshown between various action devices, e.g., air cylinders 192, and oneor more sensors, e.g., 202 and 407. These dashed lines associatecomponents which are parts of a feedback loop. For example, pressure ismaintained between a wafer and the polish table 134 by transmittingcommands from computer 103 to analog air pressure control 401 interface,which controls one or more air cylinders 192, to apply a pressurespecified in the command. The actual pressure applied to the polishtable by each carrier, e.g. 139, is then read from a pressure sensor 202via an interface 408 and adjusting commands are sent by computer 103 toair pressure control 401 to maintain the pressure at a desired level.

FIGS. 17 through 22 are flow diagrams of the wafer polishing processperformed by system 100, as controlled by computer 103. The waferpolishing process includes six basic routines which are shown in theflow diagrams and discussed in detail below. The six basic routines arestartup, input, output, load, unload, and polish. The startup routine(FIG. 17) is performed at "power on" and when new process variables areto be entered. The input routine is used to load unpolished wafers frominput cassettes 106 to the index table 117. The input routine can beperformed whenever input wafers are available, load cups 124-128 ofindex table 117 are available, and the polish assembly 132 is not usingthe index table. The output routine is performed whenever polishedwafers are available in the unload cups 119-123, an output cassette 108is available, and the polishing assembly 132 is not using the indextable 117. The load routine (FIG. 19) is performed when the load cups124-128 are full and the wafer carriers 139-143 are empty. The loadroutine is directly followed by the polish routine of FIG. 20. Theunload routine of FIG. 21 is performed whenever the carriers 139-143contain polished wafers and the unload cups 119-123 are empty. As can beseen from the foregoing, more than one of the routines can be performedat the same time. For example, during a polish routine, when thepolishing assembly 132 does not require the index table 117, both theinput and output routines can also be performed if indexing of the table117 is coordinated.

The process begins with the start-up routine (FIG. 17) when an operatorenergizes the apparatus and inserts at least one input cassette 106 withwafers, and at least one empty output cassette 108, into theinput/output module 101. Computer 103 responds in step 301 to "power on"by performing internal initialization routines of the well known type,and by initializing the system in a step 303. Such system initializationincludes reading all sensors to determine the operability of the system.Next, step 304 is performed in which process variables are entered by anoperator.

In the present embodiment, a video monitor 105 (FIG. 1) operates as atouch-screen device, permitting the entry of process variables. Otherinput devices, such as computer keyboards, could also be used. Theprocess variables entered by the operator identify certain polishingspecifics for each wafer carrier 139 through 143. For example, theoperator can enter for each wafer carrier 139-143 the pressure to beapplied on polish table 134, the rotation rate of the wafer carrier, theoscillation distance imparted by polishing arm 180, and the time thatsuch pressure is to be maintained on the polishing table. The operatoralso specifies in step 304 the rotation speed of polish table 134 andamounts of slurry to be pumped to the table. The variables specified forone polishing arm 180 may differ from those specified for other armsbut, in the embodiment which follows, it is assumed that all five wafercarriers 139 through 143 operate in accordance with the same processvariables. Computer 103 stores the process variables for each polish arm180 in different storage locations within the computer. The computer 103uses the input process variables to establish ranges of actual sensedvalues from the sensors measuring the physical variables of the process.

After the process variables have been established and stored, step 305is performed in which all five carriers 139 through 143 are raised,oscillated to the home position, and moved to the polish position. Step305 is performed by transmitting commands via air pressure control 401,to control all five air cylinders 192 to raise their connected wafercarrier 139 through 143. Completion of raising is checked by readingfive Hall effect limit detectors 407 via an interface 408. Oscillationto the home position is achieved by oscillation commands sent tooscillator servo interface 403, which applies power to the servo motors190 to rotate the carriers to the home position. Proper oscillation isthen checked by reading servo position sensors 409 of motors 190 (oneassociated with each servo motor 190) via interface 410. Next, a step306 is performed in which polish table motor 280 is sent a command viainterface 442 to achieve the rotation speed set in the processvariables. Computer 103 periodically reads the output of a rate sensor440 of motor 280 via an interface 441 to adjust the actual rotationspeed of polish table 134. Finally, the position of polish assembly 132is read from a position sensor 415 associated with servo motor 165 and,if the assembly is not in polish position, commands are sent to theservo motor 165 via an interface 417 to so move the assembly.

After placing the system 100 in a known condition, a step 307 isperformed to determine if an input cassette 108 has been loaded into aninput/output unit 101. Such a check may comprise reading by computer 103a photoelectric cell sensor 119 in input/output unit 101. When nocassette is present, an alarm or other notice may be provided tostimulate action by the operator. Alternatively, when such cassette ispresent, the process enters the input routine (FIG. 18) at a step 309which is performed to place the index table 117 in the input/outputposition, in which load cup 124 is in input position 129 adjacent to theinput gripper 115. Step 309 comprises reading a position sensor 421 ofindex servo motor 131 to identify the position of index table 117, andcommanding via interface 423 that servo motor 131 index by 36° if anunload cup, e.g. 119, is in input position 129. Alternatively, if loadcup 124 is already in the input position 129, no indexing is performed.After the input position has been established, a command is transmittedin step 311 to input robot 111, directing that a wafer be moved frominput cassette 106 to aligner 113. Proper alignment of the wafer by thealigner 113 can then be read by computer 103 to determine if the wafermovement and alignment action were successfully completed.

Upon proper alignment, a step 313 is performed in which the load cup124, at the input position 129 is raised, and the input gripper 115 iscommanded in step 315 to place the aligned wafer in the load cup. Theload cup 124 is then lowered in a step 317 and servo motor 131 iscommanded in step 318 to index by 72°. After indexing, a check 319 isperformed to determine if a computer 103-maintained count of wafersshows that five have been placed on index table 117. When fewer thanfive have been so placed, and the wafer input routine begins again atstep 311.

When all five load cups 124 through 128 contain wafers for polishing,the wafer load routine (FIG. 19) begins at step 321, after which thecarriers 139 through 143 are in the raised and home positions. A polishassembly 132 move function then begins in step 323, which includes acommand from computer 103 to clamp control 425 to unlock wedges 166 fromsupport members 168, the performance of which is checked by reading asensor 426. The polish assembly move function also includes a command toservo motor 165 to move the polish assembly 132 to the index tableposition, which movement is checked by reading position sensor 415 ofservo motor 165.

After the carriers 139 through 143 are positioned over the index table117, they are scrubbed in step 325 by being lowered and rotated againstbrush 146, while being sprayed with water from nozzles 147. The controlof water spraying is represented in FIG. 15 by a water valve controller428, which receives commands via an interface 429. In a step 327, thecarriers 139 through 143 are raised by commands to air cylinders 192 andoscillated to their maximum outward position by commands to the fiveservo motors 202. The proper raising and oscillation of the carriers139-143 is checked by reading sensors 407 and 409.

The position of the load cups 124 through 128 is checked in a step 328to establish that load cup 124 is present in the input position 129, andif an unload cup is in that position, the table is indexed by 36°. Whenthe load cups 124 through 128 are properly positioned, the load cups areraised in a step 329 by commands from computer 103 to an air cylindercontroller 431 via an interface 430. A plurality of Hall effect sensors432 are read by computer 103 to establish that proper air cylinderoperation has occurred. The cups self-align with the carriers upon beingraised, and computer 103 commands a vacuum control interface 434 tocontrol five fluid valves 435, to apply vacuum from source 438 to thesurfaces 261 of the carriers 139 through 143 via hoses to the fluidcoupling input 211 (FIG. 10). The applied vacuum secures the wafers tothe carriers 139 through 143 and the load cups are lowered to indextable 117 in a step 333. Advantageously, vacuum level checks areperformed by vacuum sensors 436 to assure that a wafer is present oneach carrier 139 through 143 before the process continues. The state ofsensors 436 is read by computer 103 via an interface 437. The wafercarriers 139 through 143 are then oscillated to the home position instep 335, and the servo motor 165 is commanded to move the polishassembly 132 to the polish position in a step 337.

Upon arrival at the polish position, the polish assembly is locked intoposition by a command to clamp control unit 425 and the polish routine(FIG. 20) begins at step 341. It should be mentioned thatcontemporaneously with polishing, the system 100 is free to perform theinput routine to prepare new wafers for polishing and/or to perform anoutput routine to be described later herein, to remove polished wafersfrom index table 117.

In step 341, the polish table rotation speed is checked by reading ratesensor 440 via an interface 441 and the rotation rate is adjusted bycommands to polish table motor 280 via interface 442. At this point,commands are sent via interface 405 (step 343) to servo motors 220 tobegin their rotation at the rate specified by the operator in the inputvariables. Also, the carriers 139 through 143 are lowered and pressedagainst the revolving polish table 134 at the specified pressure, andthe oscillation distance and speed of carriers 139 through 143 aremaintained. Advantageously, pressure sensors 202, position sensors 409,and the rotation sensors 412 are frequently read by computer 103 duringpolishing, and appropriate adjustment commands are transmitted tocarefully maintain all movements and forces within the rangesestablished for the levels specified by the operator in the inputvariables. Also, the slurry amount pumped onto polish table 134 iscommunicated to a slurry interface and the temperature of the polishtable is controlled by computer 103 communication with heat exchanger295 to maintain accurate polishing.

A timing step 349 begins to run when polishing begins and the wafercarriers 139 through 143 are raised (step 351) and their motion stoppedwhen the time variable specified by the operator is achieved. If unloadcups are then available, as is determined in step 353, the process flowproceeds to the unload routine (FIG. 21) at step 355.

In step 355, the polish assembly 132 is moved to the index tableposition, the carriers are lowered and scrubbed in a step 357, and areraised and oscillated to their maximum outward position in a step 359.In step 361, the position of index table 117 is sensed by computer 103and, if necessary, the table is rotated so that unload cup 120 is in theinput position 129. When the unload cups are in proper position, theyare raised in step 363 to align with carriers 139 through 143 and vacuumcontrol 434 is commanded to stop the vacuum at surfaces 261 to allow thewafers to drop into their respective unload cups. In actuality, it maybe found necessary to also apply a fluid, such as water, at pressure, tothe vacuum system to force the wafers from their respective surfaces261. Such fluid introduction to the system is performed by computer 103control of fluid valves 435, which are shown connected to a vacuumsource 434 and to a pressurized water source 439 in FIG. 15.

When the wafers have dropped into unload cups 119 through 123, they arelowered to the surface of the index table 117 in step 367. Carriers 139through 143 are then oscillated to the home position (step 369), loweredand scrubbed (step 371), and raised (step 373). A step 375 is thenperformed to determine if unpolished wafers are present in the load cups124-128. When wafers are present in the load cups, load routine (FIG.19) beginning at step 327 is performed.

When the polish assembly 132 has returned to the polish table, eitherwith or without wafers for polishing, and polished wafers are present inthe unload cups 119-123, the wafer output routine (FIG. 22) begins atstep 381. In step 381, the index table position is checked andcontrolled, if needed, to place an unload cup 120 in the output cupposition 131. The unload cup 120, at position 131, is then raised instep 383 by an output air cylinder 160, in response to commands fromcomputer 103 to air cylinder controller 431. Output gripper 116 is thenrotated in step 385 to the output position 131 and commanded by computer103 to grip and return the wafer from the raised unload cup to washstation 230, where a wash is performed in step 387 by a combination ofcommands to air cylinder control 431, water control 428 and steppermotor control 445. When the wash is completed as identified in a timerstep 389, a check is made in step 391 to determine whether more polishedwafers are available in the unload cups 119 through 123 of index table117, and, if so, the table is indexed by 72° and the input routinecontinues at step 383. When all unload cups 119 through 123 are empty,the output process ends.

While preferred embodiments of the invention have been illustrated, itwill be obvious to those skilled in the art that various modificationsand changes may be made thereto without departing from the scope of theinvention as set forth in the attached claims.

For example, in the described embodiment five wafers are loaded,unloaded and polished at a time, in accordance with the sameoperator-entered process variables. The process variables for eachpolish arm may be different when they are entered. Also, for certainsmall batch processes or for testing purposes, fewer than five wafersmay be a polished at a time. During the data entry phase some, e.g. 2,polish arms may be identified as idle and only three wafers rather thanfive will be placed on the index table, loaded, polished and unloaded.

What is claimed is:
 1. Apparatus for polishing a surface of a thin waferof material comprising:a polishing surface; a polishing assemblycomprising a plurality of wafer carriers for polishing wafers againstsaid polishing surface; index apparatus comprising a plurality of loadmeans equal to the number of said wafer carriers, each said load meansfor holding a wafer to be polished; first means for positioning saidwafer carriers adjacent individual ones of said load means; means forsubstantially simultaneously engaging wafers held by said load meanswith individual ones of said wafer carriers; second means forpositioning said wafer carriers in polishing engagement against saidpolishing surface; and means for moving said polishing assembly from aposition at said polishing surface to a position at said index means. 2.Apparatus in accordance with claim 1 wherein said first positioningmeans comprises means for positioning said wafer carriers over said loadmeans; andsaid means for engaging comprises means for raising said loadmeans into engagement with said wafer carriers.
 3. Apparatus inaccordance with claim 1 comprising means for automatically loadingwafers to be polished into said plurality of load means when saidpolishing assembly is positioned at said polishing surface.
 4. Apparatusin accordance with claim 3 wherein said index means is mounted forrotating said load means about a central axis thereof by predeterminedindex amounts and said load means comprises means operative insynchronism with rotation of said index means for loading wafers to bepolished into the load means one at a time.
 5. Apparatus in accordancewith claim 1 wherein said index means comprises a plurality of unloadmeans for substantially simultaneously receiving a plurality of polishedwafers from said wafer carriers; andsaid apparatus comprises means forautomatically removing polished wafers from said unload means. 6.Apparatus for polishing the surface of a thin wafer of materialcomprising:a polishing surface; a polishing assembly comprising aplurality of wafer carriers each for holding a wafer in polishingcontact with said polishing surface; index apparatus comprising aplurality of unload means equal to the number of said wafer carriers;means for moving said polishing assembly from a position at saidpolishing surface to a position at said index means; and means forsubstantially simultaneously placing wafers held by said wafer carriersinto individual ones of said unload means.
 7. Apparatus in accordancewith claim 6 comprising means for automatically removing polished wafersfrom said plurality of unload means when said polishing surface ispositioned at said polishing assembly.
 8. Apparatus in accordance withclaim 7 wherein said removing means removes said wafers from said unloadmeans one at a time.
 9. Apparatus in accordance with claim 8 whereinsaid index apparatus is mounted for rotating said unload means about acentral axis by predetermined index amounts and said removing meanscomprises means operative in synchronism with rotation of said indexapparatus for removing said wafers one at a time.
 10. Apparatus inaccordance with claim 7 wherein said means for automatically removingwafers comprises means for washing wafers after removal from said unloadmeans.
 11. Apparatus in accordance with claim 10 wherein said washingmeans comprises means for engaging a removed wafer between a pluralityof rotating brushes and means for spraying said removed wafer withsolvent.
 12. Apparatus in accordance with claim 7 wherein said means forremoving comprises means for placing removed wafers into a multi-wafercassette.
 13. Apparatus in accordance with claim 6 comprising washingmeans for washing the wafers held by said wafer carriers prior to theplacement of those wafers in the unload means.
 14. Apparatus forpolishing a surface of a thin wafer of material comprising:a polishingsurface; a polishing assembly comprising a plurality of wafer carriersfor polishing wafers against said polishing surface; index apparatuscomprising a plurality of wafer holding means at least equal to thenumber of said wafer carriers, each said wafer holding means beingcapable of holding a wafer; first means for positioning said wafercarriers adjacent individual ones of said wafer holding means; means forsubstantially simultaneously exchanging a plurality of wafers betweensaid wafer holding means and individual ones of said wafer carriers;second means for positioning said wafer carriers in polishing engagementagainst said polishing surface; and means for moving said polishingassembly between a position at said polishing surface and a position atsaid index means.